An electronic image generator, such as television set, scans a viewable image, or a sequence of viewable video images, onto a display screen by electronically sweeping an electromagnetic image beam across the screen. For example, in a television set, the image beam is a beam of electrons, and a coil generates a magnetic field or electric field to sweep the beam. An optical image generator is similar except that it scans a viewable image by sweeping a beam of light across a field-of-view. In the case of a retinal display, the optical image generator scans a viewable image onto a viewer's retina(s). In the case of a projection display system, the optical image generator scans the beam of light onto a screen, which may in turn be viewed.
FIG. 1 is a diagram of a conventional optical image-display system 10, which includes an optical image generator 12 and a display screen 14. The image generator 12 includes a beam generator 16 for generating an optical beam 18, and includes a scan assembly 20 for scanning an image onto the screen 14 with the beam. Where the system 10 is a retinal display, the scan assembly 20 scans the image onto a viewer's retina(s) (not shown). The scan assembly 20 includes a reflector 22, which simultaneously rotates back and forth in the horizontal (X) and vertical (Y) dimensions about pivot arms 24a and 24b and pivot arms 26a and 26b, respectively. By rotating back and forth, the reflector 22 sweeps the beam 18 in a two-dimensional (X-Y) pattern to generate the image on the screen 14 (or retina(s)). The scan assembly 20 includes other components and circuitry (not shown) for rotating the reflector 22 and monitoring its instantaneous rotational position, which is proportional to the instantaneous location at which the beam 18 strikes the screen 14. In an alternative implementation that is not shown, the scan assembly 20 may include two reflectors, one for sweeping the beam 18 in the horizontal (X) dimension and the other for sweeping the beam in the vertical (Y) dimension. An optical image-display system that is similar to the system 10 is disclosed in U.S. Pat. No. 6,140,979 of Gerhard, et al., entitled SCANNED DISPLAY WITH PINCH, TIMING, AND DISTORTION CORRECTION and U.S. Pat. No. 5,467,104 of Furness, et al., entitled VIRTUAL RETINAL DISPLAY, each of which is incorporated by reference.
Referring to FIGS. 1-3, the operation of a raster-scanning optical image-display system 10 is discussed. In FIG. 1 the image generator 12 scans an image through an initial pixel location X=0, Y=0 and an end pixel location X=n, Y=m, where n is the number of pixels in the horizontal (X) dimension of the image and m is the number of pixels in the vertical (Y) dimension of the image. Specifically, the beam generator 16 modulates the intensity of the optical beam 18 to form a first pixel P0,0 of the scanned image when the reflector 22 directs the beam onto the location X=0, Y=0. As the reflector 22 sweeps the beam 18 toward the location X=n, Y=m, the generator 16 periodically modulates the intensity of the beam to sequentially form the remaining pixels of the image including the last pixel Pn,m. Then, the image generator 12 flies back to scan the next video frame starting at the location X=0, Y=0, and repeats this procedure for all subsequent video frames.
Referring to FIG. 2, during the scanning of the image, the reflector 22 sinusoidally sweeps the beam 18 bi-directionally in the horizontal (X) direction at a horizontal sweep frequency fh=1/th, where th is the period of the horizontal sinusoid. FIG. 2 is a plot of this horizontal sinusoid, which indicates the position of the beam 18 in the horizontal (X) dimension versus time, where + corresponds to the right side of the screen 14 and − corresponds to the left side. As this plot shows, the reflector 22 oscillates in a sinusoidal manner about the pivot arms 24a and 24b at fh, and thus sinusoidally sweeps the beam 18 from side to side of the screen 14 at the same frequency. The horizontal sweep is bi-directional because the beam 18 is “on”, and thus generates pixels, in both the left-to-right (+X) and right-to-left (−X) horizontal directions. Although not required, fh may substantially equal to the resonant frequency of the reflector 22 about the arms 24a and 24b. One advantage of designing the reflector 22 such that it resonates at fh is that the scan assembly 20 can drive the reflector in the horizontal (X) dimension with relatively little power.
Referring to FIG. 3, the reflector 22 also, for a raster scanning system, linearly sweeps the beam 18 uni-directionally in the vertical (Y) dimension at a vertical sweep frequency fv=1/tv, where tv is the period of the vertical waveform. In the case of a raster scanning system, the slow scan waveform approximates a saw-tooth wave. FIG. 3 is a plot of this saw-tooth wave, which indicates the position of the beam 18 in the vertical (Y) dimension versus time, where + corresponds to the bottom of the screen 14 and − corresponds to the top. As this plot shows, during a vertical scan period V, the scan assembly 20 rotates the reflector 22 about the pivot arms 26a and 26b from a top position to a bottom position approximately linearly, thus causing the reflector to sweep the beam 18 from the top row of pixels (including pixel P0,0) of the screen 14 to the bottom row of pixels (including pixel Pn,m) of the screen (−Y direction). During a fly-back period FB, the scan assembly 20 quickly (as compared to the scan period V) rotates the reflector 22 back to its top position (corresponding to P0,0) to begin the scanning of a new video frame. Consequently, tv=V+FB such that the vertical sweep frequency fv=1/(V+FB). Moreover, the vertical sweep is uni-directional because the beam 18 is “on” only during the scan period V while the reflector 22 sweeps the beam from top (P0,0) to bottom (Pn,m) (−Y direction), and is off during the flyback period FB when the reflector 22 returns to its top position (P0,0).
The beam generator 16 modulates video information on the optical beam 18 and may be formed from a number of different types of light sources, such as a laser, laser diode, or light emitting diode (LED), for example. Multiple colored light sources such as red, green and blue lasers, laser diodes, or LEDs may be included in the beam generator 16 to provide colored light that is modulated with red, green, blue (RGB) information via the respective light sources. The video modulated beams from the respective light sources are combined through suitable optics and then scanned onto the screen 14 or onto the retina of a person's eye to generate a corresponding video image.
While the beam generator 16 can be formed from lasers, laser diodes, LEDs, or other suitable light sources, cost and operational characteristics may make one or more of type of light source more or less attractive depending upon the application of the image-display system 10. For example, lasers provide high intensity light for the optical beam 18, which will generate a bright image, but may be relatively expensive and require external modulation. Laser diodes can provide a high intensity optical beam 18 at a relatively low cost compared to other types of lasers. Although costing substantially less than other types of lasers, however, laser diodes are still relatively expensive.
In contrast to lasers and laser diodes, LEDs are relatively inexpensive and therefore from a cost perspective are ideally suited for applications where the overall cost of the image-display system 10 is a primary consideration. There are several different types of LEDs, with some LEDs being more specifically referred to as surface emitting LEDs (SELEDs), meaning that light is emitted perpendicular to an active region of the LED surface. The use of SELEDs, which may be referred to simply as LEDs in the present description, may be limited in the image-display system 10 for some applications due to the relatively low intensity of generated light, resulting in a low intensity optical beam 18 and an image that is not as bright as desired. For example, if the beam generator 16 were to be formed from a red, a blue, and a green SELED, the resulting image on the screen 14 may not be as bright as desired.
Many current image-display systems 10 utilize either lasers or laser diodes to generate sufficiently bright images. While lasers provide a required intensity of light, they may be relatively expensive, undesirably increasing the overall cost of the system 10.
Another type of LED known as an edge emitting LED (EELED) emits much higher intensity light from between the heterogeneous layers in the device and thus would be well-suited to use in the beam generator 16, but such EELEDs may adversely affect cost and availability of systems constructed with them.